Exploiting Biocatalysis in the Synthesis of Supramolecular Polymers - Enzymatic Polymerisation - page 136

Fig. 7 Spatial confinement of nucleation and growth as observed for Fmoc-L/L 2 system. ( a ) TEM
image of typical features observed with fibre propagation demonstrates confined fibre growth from
spherical structures. ( b ) The same process is visualised by AFM. ( c ) Right : Localised self-assembly
visualised by congo-red staining under cross-polarised light. Thermolysin is coupled to the glass
surface through PEG diamine and glutaraldehyde crosslinking. Upon immersion into the solution
of Fmoc-L/L 2 solution, self-assembly is observed in the vicinity of the immobilised enzyme. Left :
Association of the congo-red dye with the β -sheet of the fibres results in green birefringence ( I ) ,

no effect is observed on the regions without enzyme ( II ). Modified from [ 21 ]

6 Applications in Biomedicine and Nanotechnology

Supramolecular polymers are finding extensive applications in several areas of
biomedicine, which include 3D cell culture, targeted drug delivery, biosensing, in-
hibitor screening and wound healing. For example, self-assembled hydrogelating
systems of aromatic peptide amphiphiles comprising a fibrous network are suitable
as minimal mimics of the extracellular matrix and have been used to support 2D
and 3D culture of cells [ 10 , 70 , 71 ] . Using enzyme action to control formation of
these gels would provide enhanced control over stiffness, topography and chemi-
cal composition and may give rise to next generation scaffolds for the controlled
differentiation of stem cells.
An interesting recent example in the context of drug delivery of such enzyme-
triggered self-assembly includes the modification of the anticancer drug taxol
(Fig. 8 ) into a self-delivering supramolecular gel. The drug is attached covalently
to a motif that forms a self-assembled structure, which bears a molecular switch
component that is cleavable by a phosphatase (Fig. 8 ) [ 72 ] . On exposure to alkaline
phosphatase, the phosphate group of 12 is hydrolysed to give rise to 13 that forms
a hydrogel. This hydrogel can slowly release the taxol derivative 13 in aqueous
solution when it is exposed to phosphate-buffered saline. This derivative showed
similar activity to taxol in toxicology studies. Thus, this enzyme-triggered bioactive
hydrogel generates a facile strategy of dissolving otherwise-insoluble hydrophobic
drugs in aqueous solution. This example proves that small molecule drugs are ex-
cellent candidates for engineering functional hydrogels without compromising their
activities.
Enzyme-sensitive supramolecular polymers also hold promise in analytical ap-
plications such as the screening of enzyme inhibitors. A simple visual assay based
on the hydrogelation of small molecules has been developed for screening the ac-
tivities of inhibitors of enzymes like acid phosphatase. A number of inhibitors for